WO 2012077450 suggests an internal resistance measuring device that measures the internal resistance of a fuel cell while the fuel cell is supplying electric power to a load.
In order to ensure measurement accuracy, this internal resistance measuring device outputs the same AC signal to a positive electrode terminal and a negative electrode terminal of the fuel cell so that current does not leak to the load side. The amplitude of the AC signal output to each of the electrode terminals is adjusted so that a potential difference obtained by subtracting the electric potential at an intermediate terminal located between the positive electrode terminal and the negative electrode terminal of the fuel cell from the electric potential at the positive electrode terminal matches a potential difference obtained by subtracting the electric potential at the intermediate terminal from the electric potential at the negative electrode terminal. The internal resistance of the fuel cell is measured on the basis of the output AC signal that has been adjusted and the potential difference signal.